Hypergolic rocket fuels and method of controlling combustion of same



United States Patent 3,550,380 HYPERGOLIC ROCKET FUELS AND METHOD OF CONTROLLING COMBUSTION 0F SAME Kenneth C. Halliday, Jr., Port Washington, N.Y., as-

signor to the United States of America as represented by the Secretary of the Army No Drawing. Filed Jan. 18, 1955, Ser. No. 482,680 Int. Cl. F231 1/00; C06d /08 US. Cl. 60-215 16 Claims This invention relates to improved hypergolic fuels for rocket motors and the method of controlling combustion thereof. More particularly the invention provides a series of reactive mixtures for use as liquid propellants for the self-ignitable or hypergolic rocket systems which utilize concentrated nitric acid as the source of oxygen for the spontaneous combustion process.

Rocket engines may be operated by the admixture of two chemical systems which spontaneously inflame upon contact. Such so called hypergolic systems may be made in which nitric acid (white fuming or red fuming) is one of the reactants, while the other or fuel component, with which this invention is concerned, is a chemical or mixture of chemicals that are sensitive to oxidation reaction. The improved fuels and methods herein disclosed provide for suitable hypergolic fuels which in some instances are produced by the addition of amounts of oxidation sensitive chemicals to the non-sensitive chemical, methlal, and in other instances by providing fuel mixtures from combinations of allylated-bydroxy-benzenes, with an alkyl-di-hydro-pyridine and diluted with ethers, aldehydes, alcohols, or other hydrocarbons. The mixtures are so compounded as to produce one phase liquids at very low temperatures, and have other properties making them suitable for a rocket engine of the type which interacts nitric acid with such organic fuels.

During the development of the instant invention a large number of chemical components were utilized. Among these, a number which are of particular significance to the present invention are the catechols, methylal, allylated hydroxy benzenes, alkyl di hydro-pyridines, and the furan derivatives.

It is an object of this invention to provide an improved liquid hypergolic reactive propellant containing allylatedhydroxy benzenes and alkyl dihydro pyridines. The mixture may or may not contain a hydrocarbon diluent.

It is a further object of the present invention to provide a liquid hypergolic propellant fuel from methylal and a method of controlling the combustion thereof in a rocket motor utilizing concentrated nitric acid as oxidant.

I have found that excellent reactive systems having superior physical characteristics may be made by the mixture of allylated catechol or pyrocatechol and tetramethyl-di-hydro pyridine. As a diluent, I have found diallyl ether and methylal particularly desirable. As substitute diluents, the vinyl ethers, particularly vinyl n-butyl ether, n-hexane and inflammable benzene derivative compounds have proved suitable.

An important part of the present invention concerns the development of a hypergolic fuel mixture using methylal (dimethoxymethane), a relatively inexpensive chemical which is of itself not ordinarily hypergolic to nitric acid, but may be made so by the addition of compounds of the dihydric phenol group and the furan group.

Methylal, either in the pure state or containing manufacturing impurities, such as methanol, organic acids and aldehydes, upon the addition of pyrocatechol (ortho-dihydroxy-benzene) allyl-catechols, or other catechol derivatives, was found to be spontaneously inflammable upon contact with high concentration nitric acid. Methylal was found to be hypergolic also when mixed with furfuryl alcohol (alpha-furan-aldehyde); and with other furan derivatives. The spontaneous attainment of combustion 3,550,380 Patented Dec. 29, 1970 ice level of reaction, upon the intermixing of the fuel and oxidizer, has been called hypergolic ignition by the German investigators of this phenomenon, and this terminology has been generally adopted in the investigations of the reactions of these two components in the field of rocket chemistry. Many reactions are known which catch fire or explode, and the numerous chemical combinations where extensive cooling precautions are necessary, clearly prove that hypergoles are not exceptional but, on the contrary, are rather commonplace. Although hypergolic ignition of fuel and oxidizer can be accomplished with quite diverse systems, this invention is directed mainly to nitric acid hypergoles as distinguished from other oxidizers such as oxygen, air, hydrogen peroxide, etc.

In its simplest aspects, the over-all system of chemically powered rockets can be generally characterized as consisting of an oxidation-reduction pair. For the system to be hypergolic it is also necessary that the pair be mutually instable in the presence of one another, and there then results a reaction to attain a neutral condition or lower energy content. Furthermore the overall effect of the reaction sequence must be the production of exothermic heat if the reaction is to become ultimately hypergolic. Since the level of reaction is a function of the heat level, as the reaction becomes self-sustaining it will move progressively toward the higher oxidation level; and for rocket fuel purposes the more the reaction can be driven toward the higher oxidation categories, the more suitable the particular fuel system used.

The benzene ring provides the basic structure for most of the nitric acid hypergoles although this situation is probably due to availability of such chemicals as to any particular criterion of chemical activity. Benzene itself is much too stable for a hypergole, although along with sutficient contributions of external energy, combustion with nitric acid can be made to occur. The alkyl benzenes, such as toluene, xylenes, ethyl benzene, etc., despite their more reactive nature are still not adequate of themselves for a hypergolic rocket fuel. Also with regard to the alkyl benzenes, it is generally insufficient to use unsaturated side chains, such as the vinyl group; for styrene and di-vinyl-benzene are not normally hypergolic. There is, however, a reduction in stability by partial hydrogenation of the double bonds and absent the resonance stabilization of the ring a hypergole does result, e.g. vinyl-cyclohexene. The fully hydrogenated ring compound cyclo hexane although somewhat less stable then benzene is also not of itself normally of suflicient reactivity to undergo spontaneous ignition. The hydroxyl group on a benzene ring promotes oxidation; the effect is much enhanced with two hydroxyl groups.

I have found that allylated phenol and particularly the dihydric benzenes become suitable hypergoles when admixed With tetramethyl dihydro pyridine. It would be most desirable if the hydroxy benzenes having a single hydroxy group were sufficiently reactive of themselves to be come hypergolic, as the phenols are readily procurable and relatively easy to handle. However experiments have indicated that while a chemical reaction does occur with the mono-hydric benzenes, it is not generally sufficiently intense for hypergolic purposes as the reaction takes place at too low a level to attain combustion. I have found from experiments conducted to test the influence of adding a second functional group, in the form of a hydrocarbon side chain on the mono-hydric benzenes, that only minor success is attainable with phenols; and that, with the exception of allylated phenol, no simple and easily procured compounds of phenol would readily attain combustion with nitric acid. My experiments with the dihydroxy benzene compounds proved more promising, as practically all members of the homologous series of the dihydric benzenes of pyro-catechol, resorcinol and hydroquinone proved to be good hypergoles. The same difiiculty arises with these dihydric benzene compounds however, in that these are often solids and it becomes necessary therefore to use suitable solvents and dilutors to render them effective for liquid rocket fuels. Accordingly, the solubility, proper ignition lag and other physical characteristics of these compounds must be considered in selecting useful proportions from this group.

I have discovered that the above enumerated difiiculties are overcome and the allylated dihydric benzenes and allylated phenols rendered suitable to meet the requisites of desirable hypergoles when they are admixed with modifying compounds such as tetramethyl dihydro pyridine, particularly the 2,2,4,6-tetramethyl 1,2 dihydropyridine, known as tetrapyre. This substance was tested with the drop technique and ignited satisfactorily with anhydrous nitric acid, and also proved satisfactory as regards ignition lag.

The drop technique (or test) which provides good comparable information rapidly and cheaply, consists generally of placing an accurately measured amount of fuel at the bottom of a crucible and introducing a measured amount of nitric acid as a nearly continuous stream on top of the fuel. The resulting effects are noted and those mixtures which appear promising are given further attention.

The ignition lag is a measure of the time interval between the first contact of a bipropellant combination and the instant at which ignition occurs. The lag may be measured and recorded by various means, e.g. by accoustical effects (hiss, crackle, etc.); by optical effects (smoke, flame, discoloration, etc.); by changes in the electrical conductivity and presumably by other methods which would be capable of establishing the beginning and end of this interval before ignition takes place. Tetrapyre, when tested in a mixture with allyl-catechol showed a de sirable depression of ignition lag in comparison with the tests made when this substance was not present.

I found that successful liquid propellants were obtained using 25-95% allylated catechol, '125% tetrapyre and -65% or more of diluents such as diallyl ether, vinyl nbutyl ether, methylal, n-hexane and inflammable benzene derivative compounds. Of these diluents I prefer the non-hpyergolic methylal. The use of methylal is considered highly advantageous as it is relatively inexpensive and has many physical properties which enhance the desirable qualities of the resulting rocket fuel. When used with the catechols particularly, the resulting fuel displays outstanding low temperature characteristics in part due to the fact that the methylal is completely miscible and maintains adequate fluidity with 75% allyl catechol even at 40 degrees C. As methylal in substantial amounts does not ordinarily ignite with nitric acid alone, I have found that by using a catalytic compound such as the catechols, (pyrocatechol, allyl catechol, diallyl catechol and triallyl catechol), alkyl furan, pinene and tetrapyre, methylal can be effectively utilized for rocket propulsion. In view of the ready availability and inexpensiveness of methylal, its adaptability as a hypergolic rocket motor fuel is very desirable.

A highly satisfactory propellant fuel for nitric acid rocket motors, containing tetrapyre and a dihydroxy benzene, and having improved ignition lag, cheaper overall fuel cost and desirable low-temperature characteristics and excellent high-velocity flame under test, is represented by the following example in which the approximate proportions are expressed in mol percentages:

EXAMPLE I Mol percent Allylated catechol 55 Tetrapyre l0 Diallyl ether 35 Mixtures of this approximate proportion ratio produced excellent high velocity flame under test. Pyrocatechol may replace allor part of the allylated (allyl or diallyl) eatechol.

A substitution of allylated-resorcinol for allylated catechol using the same type of modifying ingredients provides a successful parallel system. Again the most desirable mol proportion ratios being approximately that of the allyl catechol system namely about 5: 1 :4 of resorcinol, to tetrapyre to diluent, respectively. In a narrower range of compositions allylated phenols may also be substituted for the catechol and used as suitable rocket fuels.

Workable hypergolic reactive systems were also found by using tetrapyre and the above mentioned diluents, and substituting allylated resorcinol allylated hydroquinone or allylated phenols for the catechols.

Examples illustrative of fuel compositions, using methylal, which in each case became spontaneously inflammable upon contact with high concentration nitric acid are the following:

Successful parallel systems using substantial amounts of methylal were also made using similar proportions of pinene, furfural, and other inflammable furan derivatives in substitution of all or a substantial portion of the catechols.

What I claim and desire to protect by Letters Patent is:

1. A hypergolic fuel for nitric acid rocket motors consisting essentially of from about 25 to allylated catechol, from about 1 to 25% 2,2,4,6-tetramethyl-1,2-dihydropyridine and from 0 to 35% of at least one diluent selected from the group consisting of diallyl ether, vinyln-butyl ether, methylal, and n-hexane.

2. A hypergolic fuel for nitric acid rocket motors comprising about 55% allylated catechol, about 35% diallyl ether and about 10% 2,2,4,6-tetramethyl-1,2-dihydropyridine.

3. A hypergolic fuel for nitric acid rocket motors consisting essentially of from about 25 to 95% allylated resorcinol, from about 125% 2,2,4,6-tetramethyl-1,2-dihydropyridine and from about 0-35 of at least one diluent selected from the group consisting of diallyl ether, vinyln-butyl ether, methylal, and n-hexane.

4. A hypergolic fuel for nitric acid rocket motors consisting essentially of allylated phenol, 2,2,4,6-tetramethyl- 1,2-dihydropyridine, and at least one diluent selected from the group consisting of diallyl ether, vinyl-n-butyl ether, methylal, n-hexane, said allylated phenol, 2,2,4,6-tetramethyl-1,Z-dihydropyridine and diluents being present in the approximate mol proportion ratio of 5:1:4, respectively.

5. A fuel for rocket motors having the property of be ing spontaneously combustible when contacted with concentrated nitric acid, consisting essentially of a mixture of methylal and allyl catechol, the latter being present in amounts of from about 20 to 80 mol percent in proportion to methylal.

6. The fuel of claim 3 in which allyl resorcinol is present in amounts of about 25 mol percent in proportion to methylal.

7. The fuel of claim 3 in which allyl resorcinol is present in amounts of about 75 mol percent in proportion to methylal.

8. A rocket motor fuel having the property of being spontaneously combustible when contacted with concen trated nitric acid, consisting essentially of a mixture of methylal, allyl catechol and 2,2,4,6-tetramethyl-1,2-dihydropyridine, said allyl catechol and 2,2,4,6-tetramethyl- 1,2-dihydropyridine being present in about 60 mol percent and mol percent respectively based on the amount of methylal.

9. A rocket motor fuel having the property of being spontaneously combustible when contacted with concentrated nitric acid consisting essentially of a mixture of methylal and alpha furan carbinol the latter being present in amounts of about 28 mol percent based on the amount of methylal.

10. The method of controlling the combustion of methylal in the combustion chamber of a nitric acid rocket motor which comprises intimately contacting nitric acid and a methylal mixture at a proportionale rate as to promote optimum combustion, said mixture having dissolved therein a catechol 2,2,4,6-tetramethyl-1,Z-dihydropyridine in the amounts of about 60 mol percent and mol percent respectively, based on the amount of methylal.

11. The method of controlling the combustion of methylal in the combustion chamber of nitric acid rocket motor which comprises intimately mixing nitric acid and methylal and etfecting said combustion by commingling said mixture with pyrocatechol whereby said methylal is hypergolically combusted, said pyrocatechol being present in an amount of about 20 mol percent based on the amount of methylal.

12. The method of controlling the combustion of methylal in the combustion chamber of a nitric acid rocket motor which comprises intimately mixing nitric acid and methylal and effecting said combustion by commingling the mixture with a catechol and 2,2,4,6-tetramethyl-1,2- dihydropyridine whereby said methylal is hypergolically combusted, said catechol and 2,2,4,6-tetrarnethyl-1,2-dihydropyridine being present in amounts of about mol percent and 20 mol percent respectively, based on the amount of methylal.

13. The method of controlling the combustion of methylal in the combustion chamber of a nitric acid rocket motor which comprises intimately mixing nitric acid and methylal and commingling with alpha furan carbinol in the combustion chamber of a rocket motor whereby said methylal is hypergolically combusted, said alpha furan carbinol being present in an amount of about 28 mol percent based on the amount of methylal.

14. A hypergolic liquid fuel for nitric acid rocket motors consisting essentially of from about 25 to of an allylated hydroxy benzene, from about 1 to 25% 2,2,4,6- tetramethyl-1,2dihydropyridine, and an inflammable, nonhypergolic organic liquid.

15. A hypergolic liquid fuel for nitric acid rocket motors consisting essentially of from about 25 to 95 of an allylated hydroxy benzene, from. about 1 to 25 of 2,2,4,6-tetramethyl-1,2-dihydropyridine and at least one diluent selected from the group consisting of diallyl ether, vinyl n-butyl ether, methylal and n-hexane.

16. A hypergolic fuel for nitric acid rocket motors consisting essentially of from about 25 to 95 of an allylated hydroxy benzene selected from the group consisting of catechols, resorcinols, hydroquinone and phenols; from about 1 to 25% 2,2,4,6-tetramethyl-1,2-dihydropyridine and at least one diluent selected from the group consisting of diallyl ether, vinyl n-butyl ether, methylal and n-hexane.

References Cited UNITED STATES PATENTS 12/1956 Malina et a1. 60-211 OTHER REFERENCES BENJAMIN R. PADGETT, Primary Examiner US. Cl. X.R. 

1. A HYPERGOLIC FUEL FOR NITRIC ACID ROCKET MOTORS CONSISTING ESSENTIALLY OF FROM ABOUT 25 TO 95% ALLYLATED CATECHOL, FROM ABOUT 1 TO 25% 2,2,4,6-TETRAMETHYL-1,2-DIHYDROPYRIDINE AND FROM 0 TO 35% OF AT LEAST ONE DILUENT SELECTED FROM THE GROUP CONSISTING OF DIALLYL ETHER, VINYLN-BUTYL ETHER, METHYLAL, AND N-HEXANE.
 12. THE METHOD OF CONTROLLING THE COMBUSTION OF METHYLAL IN THE COMBUSTION CHAMBER OF A NITRIC ACID ROCKET MOTOR WHICH COMPRISES INTIMATELY MIXING NITRIC ACID AND METHYLAL AND EFFECTING SAID COMBUSTION BY COMMINGLING THE MIXTURE WITH A CATECHOL AND 2,2,4,6-TETRAMETHYL-1,2DIHYDROPYRIDINE WHEREBY SAID METHYLAL IS HYPERGOLICALLY COMBUSTED, SAID CATECHOL AND 2,2,4,6-TETRAMETHYL-1,2-DIHYDROPYRIDINE BEING PRESENT IN AMOUNTS OF ABOUT 60 MOL PERCENT AND 20 MOL PERCENT RESPECTIVELY, BASED ON THE AMOUNT OF METHYLAL. 